Anaerobic fermentation of wastes has been in use for more than one hundred years; it is estimated "that this clean renewable source of energy could supply a large fraction of the energy used in farming operations and that if marginal lands were used to produce biomass for fuels, the entire U.S. energy demand could be met". The preceding information comes from the abstract of W. J. Jewell et al, Bioconversion of Agricultural Wastes For Pollution Control And Energy Conservation", Final Report Contract ERDA-NSF-741222A01, Cornell University, September 1976. This report discloses various systems for digestion of organic wastes including semi-continuous feed employing silo apparatus, completely mixed systems, "continuous" feed or MUNICH systems, batch load systems and plug flow systems and permutations and combinations of these, heated and unheated.
The following U.S. Pats. deal with digestion of organic wastes over a span exceeding one hundred years; some disclose gas production:
No. 39,472 to R. B. Fitts, 8-11-1863, discloses gas production in night soil treatment;
No. 39,525 to P. Eley, 8-11-1863, discloses screening and fermenting treatment of night soil;
No. 1,543,154 to C. B. Fox et al, 6-23-25 discloses fine grinding organic waste and fermenting it;
No. 1,963,581 to H. Henkelekian, 6-19-34, discloses anaerobic production of methane and thermal selection of bacteria strains;
No. 2,755,293 to D. McDonald, 7-17-56, discloses a centrifuge employed in a system having screw feed of waste organic materials being processed;
No. 3,241,943 to S. G. Bystrom, 3-22-66, discloses a gas producing sewage system treatment employing a plurality of containers in stages;
No. 3,890,129 to F. B. Chester, 6-17-75, discloses a composting device employing rotary mixing;
No. 4,040,953 to J. E. Ort, 8-9-77, discloses an anaerobic system for production of methane by a two stage system.
A. Makhijani et al, Energy And Agriculture In The Third World, Bollinger Publishing Company, date not available, but 1974 or later, discusses aspects of gas production from organic wastes including separating stages of digestion, heating considerations, vertical mixing systems, linear displacement plug flow canal systems with different stages of digestion at different points along the length of the canal, and the possibility of mixing in a canal system.
General requirements for the economic production of methane from waste biomass can be summarized with reference to the foregoing sources of information:
1. Mixing: methane production is optimized if waste slurry is continuously mixed (Bystrom patent and Chester patent);
2. Heating: methane production is optimized if the slurry mixture is maintained between 30.degree. C. and 35.degree. C. for mesophilic digestion (Bystrom patent and Chester patent), and between 50.degree. C. and 60.degree. C. for thermophilic digestion (Jewel et al publication).
3. Continuous flow: Although numerous batch type digestors have been patented and used, the labor costs and health and odor problems of batch processes make continuous flow highly desirable (Jewell et al publication).
4. Minimum Size: The material and equipment costs of a digestor scale nearly linearly with size. Thus for minimum costs, it is desirable to have limited dilution of the waste material.
5. Simplicity: The operators of digestor systems normally do not have engineering degrees to say the least. Thus the simpler the system, the better.
6. Reliability: The digestor and its supporting equipment should be capable of operating reliably under adverse conditions. Typical unintended contaminants include stones, wood chips, gravel, etc. The digestor and the charging and discharging system (pumps, et.) must be capable of handling such foreign objects with minimum risk of breakdown. (Jewell et al publication).
The digestor must also be capable of operating under variable environmental conditions, especially cold weather.
Digestor systems developed to date address one or more of the above requirements but none has provided a simultaneous solution to all requirements. Early patent literature (patents to Fitt, Eley, Fox et al, Henkelekian) describes batch type digestors which only signify that methane can be produced from farm and municipal wastes. The Fox et al patent recognizes the advantage of pregrinding waste material. The Henkelekian patent describes a batch digestor which used a portion of the methane produced to heat the tanks. The McDonald patent describes a continuous flow digestor requiring significant dilution (large size).
The Ort patent describes a continuous flow digestor (using dilute slurries) that has two stages. Continuous mixing is accomplished in the first stage. The McDonald patent describes a continuous feed system for on-farm use with several chambers. These last two systems through approaching the total solution, have several drawbacks. Both use dilute slurries to minimize pumping/mixing problems; as a consequence, system volume and therefore cost are considerably increased. They both suffer from difficult maintenance problems in the event of pump failure, and both require periodic shutdown and cleanout to prevent buildup of foreign matter.
The Makhijani publication states that a linear displacement digestor is the most prominent candidate to reduce costs. In his design, a long canal approximates plug flow (also recommended in the Cornell study). Difficulties are anticipated, however, in maintaining flow in such a system.
In summary, it is believed that no system has been previously disclosed which provides the advantages of the present system according to the objects of this invention, among which are:
to provide a system for gaseous fuel production by anaerobic digestion of waste biomass, which combines the advantages of multi-stage-digestion efficiency with the simplicity of single stage apparatus, and which affords reliability and performance suitable for long-term trouble-free use in primitive areas as well as in areas where extreme efficiency and highest quality in methane gas production are the primary goals;
to provide a system as described which is at all times substantially self-cleaning, and self-clearing of stones and other undigestible ingestants;
to provide a system as described which can process biomass or waste material efficiently at both high and low solids concentration, and in both the mesophilic and the thermophilic ranges.